Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. Some electron tube devices rely on the properties of a discharge through an ionized gas.
Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar, sound reinforcement, sound recording and reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some applications had counterparts using earlier technologies such as the spark gap transmitter or mechanical computers, it was the invention of the vacuum tube with three electrodes (called a triode) and its capability of electronic amplification that made these technologies widespread and practical.
In most applications, solid-state devices such as transistors and other semiconductor devices have replaced tubes. Solid-state devices last longer and are smaller, more efficient, more reliable, and cheaper than tubes. However, tubes are still manufactured for applications where solid-state devices are impractical.